New devs do not have a cache of the codebase in their mind, therefore their mental compiler (because that's what you're utilizing any time you code in a dynamic language) is more prone to make mistakes than devs who know what parts of the codebase to "compile" when they make modifications.

Some may argue that tests can theoretically cover all these bases. I would counter that in practice, tests almost never come close to serving as a poor man's compiler.

And of course, not even a dev who is experienced with the codebase could replace a true compiler, but I'd assume that this is deliberate trade-off, if there were competent technical decision-makers involved

I worked on a reasonably large Python codebase once, and it was definitely more frustrating than a statically typed language. I'd see that function takes an argument named "session". Great, what can I do with that? So I'd have to do `print dir(session)`, run it, and figure out how to hit that function, and then look up the documentation for that class. It's so much easier when the function actually tells you what it is expecting.

I'd also have plenty of times that I referenced a variable and forgot to create the assignment. In a static language I'd get a compile error, but in Python I'd get everything all set up to test and then get a run-time error indicating I did something stupid. Never ceased to frustrate me.

Erlang really helps you understand these, because it's dynamically typed, but the dialyzer will produce errors as a form of static optional typing.

I wrote some code a few days ago that looked like it worked. A few days later I ran the dialyzer, and it pointed out a certain code path could never be reached, because there was a comparison of an atom to an integer.

I would never have recognised that bug otherwise, because even my unit test just called that function with an integer, instead of the atom it was being called with.

Dynamic typing runs deep in what PHP/Ruby/Python offer - build it and get it running fast. Not thinking about complex types is a good thing in small projects.

But at a codebase of a certain size, static typing really helps minimise subtle bugs.